eichenbaum



Aug- 14, 1962 A. EICHENBAUM Re. 25,222A

VARIABLE BEAM ELECTRON GUN 5 Sheets-Sheet 1 Original Filed Nov. 20, 1958l 35/1 l i 7 l a' if@ zii/w Faux/M Hier/wai INVENTOR. ERIE L. E1CHENBHUM Aug- 14 1962 A. 1 EICHENBAUM Re. 25,222

VARIABLE BEAM ELECTRON GUN +/a +5 -5 -a -1'5 -za canfVieafA/CEH n/Vivai/Vas co/vVmi/Vcs Mali M/mesif #amm/zin pari/Vn L INVENTORI ERIE L.EIEHENBHUM BY M Aug. 14, 1962 A. 1 EICHENBAUM VARIABLE BEAM ELECTRON GUN5 Sheets-Sheet 3 Original Filed Nov. 20, 1958 INVENTOR. ERIE L. E1LHENBHUM United States Patent Qfice Re. 25,222 Reissued Au-g. 14, 196225,222 VARIABLE BEAM ELECTRON GUN Arie L. Eichenbaum, Levittown, Pa.,assignor to Radio Corporation of America, a corporation of DelawareOriginal No. 2,996,640, dated Aug. 15, 1961, Ser. No.

775,195, Nov. 20, 1958. Application for reissue Feb.

26, 1962, Ser. No. 176,500

9 Claims. (Cl. 315-15) Matter enclosed in heavy brackets appears in theoriginal patent but forms no part of this reissue specification; matterprinted in italics indicates the additions made by reissue.

This invention relates to electron guns, and particularly to a novel andimproved means for controlling the trajectory and shape of high densityelectron beams.

High density electron beam-type amplifiers and oscillators generallyrequire electron guns which provide electron beams having uniformdensity, laminar rectilinear fiow, and a lprescribed divergence orconvergence angle. As presently known, most electron beam-type microwaveamplifiers, such as the traveling wave tube and the klystron, employ aPierce type of electron gun. The Pierce type electron gun uses shapedelectrodes having a specified geometry to control the iiow and focusingof the electrous in the tube thus producing a uniform rectilinearelectron flow. According to the Pierce principle, an ideal electrodearrangement provides a Zero equipotential surface disposed atapproximately a 671/2 angle with respect to the normal to the cathodeemitting surface at the periphery thereof. (Refer to Vacuum Tubes by K.R. Spangenberg, 1948, pages 450-458.) The predetermination of theelectrode arrangement in the electron gun generally fixes the shape ofthe electron beam so that the beam is initially convergent or divergentat a fixed angle.

It is desirable to have an electron gun which allows simple controlexternal to the electron tube for shaping an electron beam. An externaladjustable control to vary the initial divergence or convergence of anelectron beam may also be advantageous in reducing the noise figure ofan electron tube.

An object of this invention is to provide a novel and improved electrodeconiiguration for an electron gun.

Another object of this invention is to provide an improved electron gunhaving a control external to the electron tube which can vary theinitial divergence or convergence of the electron beam.

Another object is to effect a reduction in the noise figure of anelectron tube.

According to the invention, a beam-focusing electrode is positionedbetween the conventional beameforming and accelerating electrodes of aPierce-type electron gun. The configuration of the electrodes is suchthat when'the voltage of the beam-focusing electrode is varied, theinitial angle of divergence or convergence of the electron beam iscontrollably changed. Means external to the electron tube are providedfo-r adjusting the voltage of the beam focusing electrode thus affordinga convenient control for modifying the shape of the electron beam.

The invention will be described in greater detail with reference to thedrawing wherein:

FIG. 1 is a transverse sectional view of an electron gun according tothe invention, but in half, showing the configuration of electrodesrequired for an electron beam which may be varied from 10 convergent to20 divergent;

FIG. 2 is a transverse sectional view of an electron gun, cut in half,showing the same configuration of electrodes as in FIG. 1, includingzero equipotential surfaces for several convergence-divergence angles asmeasured in an electrolytic tank;

FIGS. 3 and 4 are graphs including curves indicating the voltagesapplied to the beam-forming and beam-focusing electrodes of the electrongun to produce a particular convergence-divergence angle of the electronbeam;

lFIG. 5 is a transverse sectional view of an electron gun, cut in half,showing the configuration of electrodes required for an electron beamwhich may be varied from 10 convergent to 30 convergent; and

FIGS. 6 and 7 are partial sectional views of electron tubes positionedin a magnetic field and incorporating electron guns similar to thoseillustrated in FIGS. l and 5, respectively.

Similar elements are indicated by similar reference charactersthroughout the drawing.

An embodiment of this invention is shown in FIG. 1, in half section,wherein an electron gun comprises a cathode electrode 11, anaccelerating electrode 13, a beam-forming electrode 15, and abeam-focusing electrode 17. The various elements of the electron gunstructure are supported as a `unit within a tube envelope. The cathode11 consists of a narrow cylindrical portion 19 joined with a broadercylindrical portion 21, the narrow portion 19 being solid whereas thebroad portion 21 is hollow. The solid cylindrical cathode portion 19flares outwardly at one end to form a frustoconical portion 2.3 havingas a base a transverse surface which is coated with a thermionicemitting material to provide a cathode emitting face 25. The emittingface 25 lies in a plane substantially perpendicular to the longitudinalaxis of the cathode 11, hereinafter designated the Z axis. A heaterfilament (not shown) is positioned within the hollow cylindrical section21 for supplying heat to the cathode 11 to cause emission of electronsfrom the cathode emitting face 25. During operation of the tube, a highdensity electron beam which may be initially convergent or divergent isprojected along the Z axis from the emitting face 25.

The accelerating electrode 13 is an annularelectrode in the *form of adisk having a centrally located aperture 29, which is larger in areathan the `mitting face 25, to allow passage of the electrons emittedfrom the cathode face 25. The accelerating electrode 13 is spaced fromthe emitting face 25, preferably at a -distance greater than thediameter or width of the face 25, along the Z axis in the direction ofelectron beam, and is mounted concentric to the Z axis. The position andthe voltage of the accelerating electrode 13 are factors in determiningthe shape of the electron beam and the total current drawn.

Fixed closely adjacent to the narrow portion 19 of the cathode 11 is thebeam-forming electrode 15 which is generally conical, in the exampleshown in FIG. 1, and has a relatively small central aperture 31. Theelectrode 15 is so positioned that its aperture 31 is close to butspaced behind the cathode face 25, as shown in FIG. 1. The beam-'formingelectrode 15 is shaped in such a manner as to produce a required zeropotential surface adjacent to the cathode emitting face 25.

According to this invention, a beam focusing electrode 17 having arelatively large aperture 33 is mounted coaxially with and intermediatethe accelerating electrode t3 and the beam-forming electrode 15. Thebeam-focusng electrode 17 is preferably an annular disk which is)ositioned substantially in the plane of the cathode emiting face 2S.Small changes of Voltage applied to the :lectrode 17 shifts the zeroequipotential surface substan- :ially thus varying the electron beamshape.

A source of direct current voltage 34 provides suittble potentials toeach of the electrodes. In the pre- ;'erred embodiment of the invention,the cathode 11 and he accelerating electrode are maintained at constantfoltages, whereas the voltage applied to the beam-formng electrode 15and beam-focusing electrode 17 may )e varied.

In FIG. 2 there are shown geometrical relationships )f various zeropotential surfaces for corresponding confergence-divergence angles ofthe electron beam that were letermined by measurements made in anelectrolytic ank, which method is well known in the art. Thus, for Lparallel rectilinear beam having a zero convergencelivergence angle, azero equipotential surface 35 forms L 671/2 angle relative to the Z axiswithin a beam radius rom the periphery of the cathode emitting surface25. IVith the electrode configuration illustrated, measurements ndicatethat at distances of about 3 lbeam radii from the eriphery of thecathode face 25, the zero equipotential urface 351 for zeroconvergence-divergence becomes arluate and denes a larger angle than671/2 with respect o the Z axis.

The geometrical arrangement of the electrodes shown n FIGS. 1 and 2creates zero equipotential surfaces coresponding to electron beam anglesof convergence-di- 'ergence between 20 divergence and 10 convergencelepending upon the potentials of the beam-forming elecrode 15 and thebeam-focusing electrode 17. As illusrated in FIG. 2, the zeroequipotential surface for a 20 liver-gent beam is represented by theline denoted as 20. The beam-focusing electrode 17, in this embodinentof electron gun shown by way of example, is lolated close to the zeropotential surface for an electron earn having a 10 divergent How denotedas l0. It s understood that the beam-focusing electrode 17 may epositioned along any other equipotential surface proided that it islocated in the approximate center of the .esired convergence-divergencerange, for example, from divergence to 10 convergent as shown in iFlG.2. The ero equipotential surface for a 10 convergent beam is ,enoted asin FIG. 2.

The graph of IFIG. 3 illustrates the voltages which are pplied to thebeam-forming electrode and beamocusing electrode 17 of the configurationof FIGS. l and to produce a beam within the range of 10 converence todivergence, when the cathode is maintained t zero potential. In theexample shown in FIG. y1, with voltage on the accelerating electrode 13of one volt posiive with respect to the cathode, the beam-focusingelecrode 17 is set at about .l0 volt negative relative to round, and thebeam-forming electrode is set at approximately .25 volt negativerelative to ground to produce a eam having a 10 divergence angle. Theordinate values of the graph of FIG. 3 represent normalized poentials.

VBF

VAcc

vhere for a given angle of convergence-divergence, VBF- s the 'voltageapplied to the beam-forming electrode .5 as indicated by a curve a, VBFis also the voltage .pplied to the beam-focusing electrode 17 as read onurve b, and VAC@ is the voltage of the accelerating elecrode 1,3. Thus,any change in the accelerating electrode foltage requires a proportionalchange in the beam elecrodes 15 and 17 for the same electron beam flowangle.

It is noted that the voltage applied to to the beam-formng electrode 15is varied to a smaller degree in relation o the changes in potentialapplied to the beam-focusing electrode 17 While holding the beam-formingelectrode 15 at a constant potential of about .30 volt negative relativeto the cathode, measurements made with the electrolytic tank indicatethat variations in potential from about .53 volt negative to about .22volt positive applied to the beam-focusing electrode 17 change the angleof the electron beam from 10 convergent to 20 divergent as shown bycurves c and d in FIG. 4.

Other combinations of electrodes and applied potentials may be used fordifferent desired ranges of electron beam convergence-divergence. As anexample, a configuration of electrodes such as disclosed in FIG. 5 maybe employed, wherein the cathode emitting surface 37 is concave and aidsin converging the electron beam, to produce a high density electron beamin the range of 10 to 30 convergence. The beam-focusing electrode 17 andthe beam-forming electrode 15 have their apertures close to the plane ofthe emitting surface, as in FIGS. I and 2, but are set at smaller anglesrelative to the [Z] longitudinal axis than in the arrangement ofelectrodes shown in FIGS. I and 2 for the case of 10 convergence to 20divergence. `[However, in every application of the invention] Moreover,ns shown in FIG. 5, the zero [potential] equipotential surface for a[particular electron] beam angle of 10 convergence [-divergence] islocated between the 'beam-forming electrode 15 and the beam-focusingelectrode 17, instead of between electrodes J3 and 17 as in FIG. 2.

To provide an electron gun having a maximum divergence angle greaterthan 20, the beam-forming electrode 15 is located further away from thebeam-focusing electrode 117. If a greater convergence angle than 10 isrequired, the beam-focusing electrode 17 may be located along the 0 or 5convergence zero potential surface. A total range of 40 divergence to 10convergence may then be covered with an electron gun, according to theinvention, by the simple adjustment of the potentials of thebeam-forming electrode 15 and the beam-focusing electrode 17 By the useof a gun structure that produces a beam that is divergent in the regionnear the cathode 11, the noise iigure of the tube is substantiallyreduced.

IIn microwave beam tubes, the beam from a magnetically shielded Piercetype gun normally enters a strong axial magnetic field near a pointWhere the radius of the beam is a minimum, so that magnetic focusingforces largely determine the beams subsequent behavior. The initialformation of the electron beam is determined principally by the electricfields which are established according to the shapes, potentials, andarrangement of the gun electrodes.

When the electrons of a beam which enter the magnetic focusing eld havea transverse velocity component, the Brillouin tiow is affected and apoorly formed, scalloped beam may result. The Brillouin iiow is definedas a magnetically focused electron stream in which all electrons areconsidered to be rotating in circles concentric with the axis withuniform angular velocity and have the same axial component of velocity.Forces acting on electrons are in balance at all radii. However, arandom transverse electron velocity distribution in the beam results ina ilovv condition differing appreciably from this ideal picture, and thescalloping effect occurs.

The electrode configurations described herein aiord the advantage ofproviding proper Brillouin flow when utilized in an electron tube whichis positioned in a magnetic field. In each of FIGS. 6 and 7, a magneticeld is shown provided by an external magnetic coil 39 surrounding a tubeenvelope 40. The gun comprises an accelerating electrode E4-1 in theform of a cylindrical cup of magnetic material having a central aperture43. The cup 41 encloses the cathode 11, the beam-forming electrode 15and beam-focusing electrode 17 to serve as a magnetic shield therefor.The fringing magnetic field B produced by the coil 319v in the aperture43 deliects the electrons which pass through the aperture y43 of theaccelerating electrode 41 into spiral paths to produce a constantdiameter beam with Brillouin flow.

A feature of this embodiment incorporating the invention is that thelocation of the magnetic shield or accelerating electrode 41 is notcritical because the plane of minimum ybeam diameter may be shifted byvarying the potentials of the beam-forming electrode and thebeam-focusing electrode 17, the apertures of which are close t0 theplane of the emissive surface of cathode 11, as in the other embodimentsof the invention. Also, by employing the desired gun design andpotentials, proper boundary conditions along the edge of the beam at thecathode are established resulting in uniform emission and awell-collimated beam. Since the electrons of the beam arrive at theaperture of the accelerating electrode with a small radial velocity as aresult of the adjusted flow angle, a laminar ilow of electrons with auniform current density over the cross section of the electron beam isprovided upon entry into the magnetic field B causing a consequentreduction in scalloping. Also, since there is a laminar rectilinear ilowof electrons towards the target, a magnetic field of lower intensity isrequired for focusing. Therefore, a smaller magnet of less weight andrequiring less power may be employed to provide a weaker magnetic ield.

It is understood that the invention is not limited to the geometries ofthe electrodes shown by way of example, but is applicable to variouscombinations of electrodes employed in an electron gun which includes abeam-forming electrode with an aperture located behind a cathodeemitting face and a beam-focusing electrode having an aperture[substanitally in] close to the plane of the cathode emitting surfacewith means external to the tube for varying the potential of thebeam-focusing electrode.

What is claimed is:

l. An electron gun for producing and shaping an electron beam comprisinga cathode having an electron emitting surface with a predeterminedwidth, a beam-forming electrode and a beam-focusing electrode, saidelectrodes each having a single aperture in a plane located adjacent tosaid emitting surface at a distance from said surface less than one halfof said width, and terminal means for applying diiferent variablevoltages -to said electrodes to vary the convergence-divergence angle ofthe beam.

2. An electron gun contained within an electron tube for projecting avariable-shape electron beam along a predetermined axis comprising acathode having an emissive surface, -a plurality of spaced aperturedelectrodes including a beam-forming electrode, a beam-focusing electrodeand an accelerating electrode coaxially mounted relative to said axisand spaced from said emissive surface, the aperture of saidbeam-focusing electrode being substantially in the plane of saidemissive surface and positioned between the other two of said aperturedelectrodes, and terminal means external to said tube for applyingdifferent potentials to said electrodes, and means for varying thepotential applied to said beam-forming and beamfocusing electrodes toVary the convergence-divergence angle of said electron beam.

3. An electron gun for projecting an electron beam comprising a cathodeelectrode having an emitting face, a beam-forming electrode having anaperture therein, a beam-focusing electrode having an aperture therein,the planes of said apertures being spaced from said cathode emittingface at distances less than one half of the width of said emitting face,an accelerating electrode positioned in front of and spaced from saidcathode face at a distance greater than said width, means for applyingdifferent potentials to said electrodes, and mean-s for separatelyvarying the potentials of the beam-forming and beamfocusing electrodesto vary the convergence-divergence angle of the electron beam.

4. An electron gun in an electron tube for producing a high densityelectron beam comprising a cathode electrode having an emitting face, anaccelerating electrode spaced from and in front of said emitting. face,a beamforming electrode disposed behind said emitting face, abeam-focusing electrode positioned between said accelerating andbeam-forming electrodes, means for applying potentials to saidelectrodes, and means external to said tube for varying the potentialapplied to said beam-focusing electrode whereby the angle o-fconvergence or divergence of said electron beam is changed.

5. An electron gun for providing an electron beam comprising a cathodeelectrode havin-g an emitting face for producing an electron beam, abeam-forming electrode having an aperture closely spaced from `anddisposed behind said cathode emitting face, an accelerating electrodedisk positioned in front of said cathode emitting face having anaperture larger in diameter than said beam-forming electrode aperture, abeam-focusing electrode positioned between said beam-forming andaccelerating electrodes and having an aperture substantially in theplane of said emitting face, means for applying voltages to saidelectrodes, and means for varying the voltage applied to saidbeam-focusing electrode whereby Ithe convergence-divergence angle ofsaid electron beam is varied.

6. An electron `gu-n for projecting a variable-shape electron beam alonga predetermined axis comprising a cathode having an emissive surfacetransverse to said axis, an apertured accelerating electrode spaced fromsaid cathode, an annular beam-forming electrode having an aperture, anannular beam-focusing electrode between lsaid accelerating electrode andsaid beam-forming electrode and lying substantially in the transverseplane of said emissive surface, whereby said gun can be controlled toprovide either convergent, divergent or parallel flow in the regionbetween said cathode and said accelerating electrode by the applicationof suitable voltages to said beam-forming and said beam-focusingelectrodes.

7. An electron gun for producing an electron beam comprising a cathodeelectrode having an emissive surface of approximately .025 inchdiameter, the longitudinal axis of said cathode electrode pas-singthrough the approximate center of said surface at a normal to the planeof said surface, an accelerating electrode having an aperture spaced infront of said emissive surface at a distance of about .041 inch, abeam-forming electrode having an aperture surrounding said cathode, theplane of said beam-forming electrode aperture parallel to and spacedfrom said emissive surface at about .003 inch, a beamfocusing electrodedis-posed between said accelerating electrode and said beam-formingelectrode having an aperture, the plane of said beam-focusing electrodeaperture positioned substantially in the plane of said emissive surface,means for applying voltages to said electrodes, and means to Vary thevoltages of the beam-forming and beam-focusing electrodes therebychanging the shape of said electron beam.

8. A variable shape electron beam producing device comprising -anelectron gun having a cathode elect-rode with an electron emittingsurface, a plurality of electrodes having apertures spaced closely fromsaid cathode emitting surface, means for applying variable potentials tosaid plurality of electrodes, yto vary the convergence-divergence angleof the beam, a cup-shaped accelerating electrode of magnetic materialmagnetically shielding said electrodes and having an aperture alignedwith said first named apertures, and means for providing a magnetic eldalong the axis of said ydevice for producing a uniform diameter electronbeam having Brillouin ow in the region beyond the aperture in saidcup-shaped electrode.

9. An electron gun for projecting a variable-shape electron beam along apredetermined axis comprising a cathode having an emissive surfacetransverse to said axis, an apertured accelerating electrode spaced fromand in -front of said cathode, an annular beam-forming electrode, and anannular beam-focusing electrode between said accelerating electrode andsaid beam-forming electrode, the apertures in said annular beam-formingand beamfocusing electrodes being disposed close to the plane of saidemissive surface, whereby said gun can be controlled to produce variousdegrees of convergence or divergence,

trodes.

References Citedin the le of this ptent l 5 or the original patentUNITED STATES PATENTS Knoll Oct. 5, 1937 8 Pierce Dec. 30, 1941 FloryJuly 13, 1943` Hae `May 21, 1946 -Weirner Nov. 2, 1948 yLinder Sept. 11,1951 Field July 23, 1957 Brewer Oct. 29, 1957

